Avascular necrosis (AVN) of the hip has occasionally been reported in the setting of HIV infection. After we diagnosed AVN in two patients in May 1999, we became concerned that HIV-infected patients may be at higher risk for developing AVN than previously recognized. To examine this, we undertook a magnetic resonance imaging (MRI)-based study of asymptomatic HIV-infected patients to determine the prevalence of hip AVN in our clinic population. 15 of 339 (4.4 percent) patients had evidence of AVN by MRI: six had bilateral and nine had unilateral involvement. None of 118 HIV-negative volunteers had MRI evidence of AVN. Prospectively performed physical examinations did not distinguish HIV-infected patients with AVN from those without. Patients with osteonecrosis were more likely than patients without osteonecrosis to have used corticosteroids, lipid-lowering agents, and testosterone, and were more likely to routinely exercise by bodybuilding. Thus, HIV-1 infection should be included among medical conditions that predispose to the development of osteonecrosis. Long-term followup of this cohort is underway to determine the natural history of symptomatic and asymptomatic lesions, and the incidence of new lesions.